A programmable logic switch is a device that controls switching on and off of a logic switch in accordance with stored data. Normally, programmable logic switches are used in field programmable gate arrays (FPGAs) that require reconfiguration of logic operation circuits and wire circuits.
In each programmable logic switch used in FPGAs, a volatile memory such as a SRAM is used as a memory. Each SRAM includes a flip-flop unit that retains data, and an NMOS transistor that has the function to transmit information from bit lines to the flip-flop unit or retain the information in accordance with the bias conditions of word lines. Based on the information retained in the SRAM, switching on and off of the pass transistor is controlled. However, each SRAM is a volatile memory, and therefore, stored data is lost once the power is turned off. When the power is turned on again, it is necessary to read data from a memory region that is formed separately from the programmable logic switch, and to write information into the SRAM. Such a programmable logic switch including a SRAM has the problem of the circuit size of the SRAM and the problem of an increase in the circuit size due to the memory region required to be formed separately from the programmable logic switch. Those problems are particularly conspicuous in FPGAs that include a large number of programmable logic switches.
Nonvolatile programmable logic switches using nonvolatile memories have been suggested. For example, there has been a nonvolatile programmable logic switch including at least three transistors: a pMOS-type nonvolatile memory and an nMOS transistor are connected in series, and a switch device (an nMOS transistor) is combined with the pMOS-type nonvolatile memory and the nMOS transistor connected in series. The potential to be transferred to the gate electrode of the pass transistor is determined in accordance with the information to be written into the nonvolatile memory, and the switch device is controlled to be in an ON state or in an OFF state in accordance with the transferred potential.